A dispersion of metal nanoparticles refers to a solution wherein highly conductive metal nanoparticles such as copper, nickel, cobalt, silver, etc. are dispersed. Metal nanoparticles with a diameter 100 nm or smaller exhibit unique properties including decreased melting point and sinterability at low temperature because of increased surface energy. Especially, since silver nanoparticles with a size of 30 nm or smaller can be sintered at low temperature of 150° C. or below, they may be used to print circuits on polymer materials. It is reported that the dispersion of metal nanoparticles may be prepared into ink to form wirings by a direct-write noncontact technology such as inkjet printing in order to produce printed circuit boards (PCBs) and flexible printed circuit boards (FPCBs).
With regard to production of such ultrafine nanoparticles, a method of using C6-C20 fatty acids to control the particle size is well known. However, although the metal nanoparticles prepared by this method are dispersed well in hydrophobic solvents such as toluene, they are not dispersible in aqueous-based solvents. With a metal nanoink prepared from such hydrophobic solvent-dispersible metal nanoparticles, it is difficult to form fine wirings through inkjet printing because of small contact angle with the substrate as well as high spreadability. Further, because volatile organic solvents are generally used as a dispersion medium, the cost is high and there is a risk that the toxic organic solvent may be evaporated.
Meanwhile, when a reducing agent is used to prepare metal nanoparticles in aqueous-based solvents, particle recovery rate is low and it is difficult to form ultrafine nanoparticles with a particle size 30 nm or smaller of uniform particle size distribution.
To solve these problems, ultrafine metal nanoparticles with a particle size 30 nm or smaller may be prepared in a hydrophobic solvent and then their surface may be hydrophilically treated so that they may be dispersed in aqueous-based solvents. However, when the metal nanoparticles are directly added to an aqueous-based solvent for dispersion, the metal nanoparticles are not completely dispersed in the aqueous-based solvent but are precipitated because of their hydrophobicity. Thus, it is difficult to prepare a high-concentration dispersion of 40 wt % or more. Moreover, copper nanoparticles are extremely difficult to make hydrophilic because the surface is easily oxidized upon contact with an aqueous-based solvent.
Metal nanoparticles capped with fatty acid are associated with the problem that the capping material is not removed well even at high temperatures because of the strong binding between the nanoparticles and the capping material. Because the capping material has a high boiling point, it may remain even after sintering of the metal nanoparticles and thereby increase the resistance of the wiring. Recently, polymer films such as polyimide (PI) are frequently used in FPCBs. In this regard, a dispersion of metal nanoparticles that can be sintered at low temperature of 250° C. or below and thereby exhibits low specific resistance is required. For this, it may be necessary to use a capping material evaporable or removable at low temperature or to effectively remove the high-boiling-point capping material.
Korean Patent Publication No. 10-2008-0032814 describes a method of reacting metal nanoparticles with an alcohol- or thiol-based solvent and modifying the surface of the metal nanoparticles by adding a capping material having a hydrophilic group for application as a dispersion for inkjet printing. With this method, nanopowders produced in large scale through the existing plasma process may be dispersed in an aqueous-based solvent. However, it is restricted to metal nanoparticles bound to amorphous carbon layer. Further, when the particles are dispersed in alcohol, the hydrophilic groups may not be attached on the entire surface of the particles if they remain as agglomerates without being separated. Moreover, when the particles are heated in alcohols or acids, the surface of the particles may be oxidized.
Korean Patent Application No. 10-2004-0070303 discloses a method of synthesizing hydrophobic particles in an organic solvent through pyrolysis and then making them hydrophilic. The method involves mixing a solution of water-insoluble particles with a solution of water-soluble ligands such that the surface of the water-insoluble nanoparticles is substituted with the water-soluble ligands, and then forming a shell by crosslinking the ligands. This method avoids the use of a toxic organic solvent. Further, since the particles are not directly contacted with the aqueous-based solvent, oxidation of the particles may be prevented. And, the rigid shell surrounding the particles results in improved water dispersibility. However, according to the example presented therein, only about 3 g of particles are produced with 1 L of a reaction solvent. Thus, it is difficult to produce the water-soluble particles in large scale. In addition, the capping material, i.e. the water-soluble ligand including fatty acid, may not be eliminated during sintering because of large thickness.
Korean Patent Application No. 10-2005-0087083 describes a method of synthesizing hydrophobic particles with uniform particle size in an organic solution and making them hydrophilic through chemical surface modification. However, this method is not environment-friendly since a toxic solvent such as chloroform is used. Besides, a strong covalent bond between the hydrophilic ligand 3-mercaptopropionic acid and the metal nanoparticles make it difficult to remove the organic material through sintering.